Copper salts and to a lesser degree its metallic form, have been used for many years as antimicrobial or anti-fouling additives in paints, surface coatings, and even swimming pools. In particular, many marine paints and antifouling surfaces employ some type of copper compound. Interest in antimicrobial surface coatings has recently increased due to the broad emergence of antibiotic resistant strains of microbes such as MERSA and tuberculosis. While there has been great interest in silver salts and silver nanoparticles due to their well know antimicrobial activity, copper may also serve a similar function, is much less expensive, and forms coatings that have less color and are often nearly colorless due to the much weaker surface plasmon resonance and the lack photosensitivity that causes most silver salts to darken with age and light exposure. There is also some evidence that copper may be more effective for some microbial strains such as fungi or may compliment silver or even traditional antibiotics, possibly synergistically, to provide a much enhanced and broader spectrum antimicrobial response [see for example, Ruparelia et al. Acta Biomaterialia 4 (2008) 707-716]. Low levels of copper can be highly effective antimicrobial agents with low toxicity to humans and the environment.
A wide variety of methods for preparing copper nanoparticles are known and include electrochemical methods, high vacuum copper sputtering and deposition, and chemical methods [see for example, Ramyadevi et al., Materials Letters 71 (2012) 114-116].
Certain polymer composites with copper nanoparticles are described in the literature. For example, it is known to form copper nanoparticles by hydrogen reduction of copper ion in cellulose acetate to be used for chemical catalysis [see for example, Shim et al., Bull. Korean Chem. Soc. 2002, Vol. 23, No. 4, 563-566]. A strong antifungal response was found using copper nanoparticles synthesized by an electrolytic method and then dispersed in poly(vinyl methyl ketone), poly(vinyl chloride), or poly(vinylidene fluoride) in the presence of acetonitrile or tetrahydrofuran solvent according to Cioffi et al., Chem. Mater. 2005, 17, 5255-5262. Rod-shaped copper nanoparticles were synthesized by sodium borohydride reduction in an aqueous poly(vinyl pyrrolidone) solution according to Harada et al., Journal of Physics; Conference Series 61 (2007) 394-398). Poly(vinyl pyrrolidone) was used as the polymeric stabilizer in a synthesis of low dispersity copper nanoparticles in diethylene glycol with sodium hypophosphite as a reducing agent according to Park et al., Journal of Colloid and Interface Science 311 (2007) 417-424. Extremely small (approx. 2 nm) copper nanoparticles stabilized by thiolate functionalized polystyrene have been synthesized in tetrahydrofuran solvent with hydrazine as a reducing agent. This material appears to be coextruded with conventional polystyrene at 190° C. due to the exceptional stability of the copper nanoparticles [see Bokern et al., Polymer 52 (2011) 912-920]. Copper nanoparticles in the size range of 10 nm to 40 nm were prepared in an aqueous solution of about 3 weight % poly(vinyl pyrrolidone) without a disclosed reducing agent [Mishra et al., Open Journal of Acoustics, 2011, Vol. 1, No. 1, 9-14]. An aqueous preparation of copper nanoparticles dispersed in poly(vinyl pyrrolidone) (PVP) using sodium hypophosphite as the reducing agent at pH 1 was considered to have exceptional stability from the strong PVP interaction in Lai et al., Journal of Applied Polymer Science, 128: 14543-1449, 2013. Moreover, copper particles were prepared in glycerol at temperatures up to 150° C. using either PVP or poly(vinyl alcohol) (PVA) as the stabilizer and hydrazine as the reducing agent (see for example, Cao et al., Journal of Experimental Nanoscience, DOI: 10.1080/17458080.2013.848298). Another aqueous preparation employs a combination of poly(acrylic acid) with the cationic poly (1,2-dimethyl-5-vinylpyridinium methylsulfate) and hydrazine borane as the reducing agent to produce a range of copper nanoparticle sizes at pH below about 3 and depending on the polymer composition (see for example, Litmanovich et al., Polymer Science, Ser. B, 2014, Vol. 56, No. 3, 326-344). Preparation of copper nanoparticles in natural polymer chitosan using hydrazine as a reducing agent is discussed by Usman et al., Molecules 2012, 17, 14928-14936.
The preparation of copper compounds with aqueous water-soluble polymers is described in U.S. Application Publication 2011/0206753 (Karpov et al.), for example, copper oxalate or copper hydoxycarbonate in the presence of poly(carboxylate ether) polymer (Sokolan from BASF). U.S. Application Publication 2010/0119829 (Karpov et al.) describes copper or zinc oxide and hydroxide nanoparticles formed in the presence of acrylic carboxylate polymers. U.S. Application Publication 2013/0171225 (Uhlmann et al.) describes nanoparticulate copper salts such as copper iodide prepared in hydrophilic polymer emulsions including acrylics and poly(vinyl pyrrolidone). U.S. Application Publication 2012/0302703 (Greiner et al.) describes the formation of copper nanoparticles enveloped in aromatic sulfide modified non-water-soluble polymers such as polystyrene and poly(methyl methacrylate) where polymerization and nanoparticle formation is done sequentially in the same vessel.
Within the last 5 years, researchers have described reduction in biofouling and microbial colony formation by use of very specific surface patterns embossed or imprinted in a polymer layer (see for example, Magin et al. Biomacromolecules 2011, 12, 915-922; Magin et al. Biofouling Vol. 26, No. 6, August 2010, 719-727; Carman et al., Biofouling, 2006 Vol. 22 No. 1, 2006, 11-21; U.S. Pat. No. Application Publication 2010/0226943, and U.S. Pat. Nos. 7,650,848 and 7,143,709, all Brennan et al.). Such patterns typically have minimum feature dimensions of about 2 μm and a specific pattern known as the SHARKLET™ AF pattern has been shown to have desirable overall performance compared to simpler patterns with similar dimensions. The polymer used most frequently in such patterns is a polydimethylsiloxane (PDMS) type polymer, although acrylic polymers have also been demonstrated.
While there are numerous polymer-copper complexes described in the art, there remains a need for polymer-copper complexes that are water-soluble and water-coatable, but which also can be crosslinked with UV light to become water-insoluble and highly durable after application to a substrate. There is a need for such polymer-copper complexes that can be readily used in antimicrobial compositions or articles.
There is the further need to provide water-soluble polymeric complexes that contain reducible copper ions that are readily reduced in the polymeric complexes, before or after the polymers are crosslinked. It would be desirable to form copper nanoparticles in uniform coatings or metallic patterns in the size range of 1 to 500 nm using such materials.